Electron discharge device, including fluorescent screen



3 1943- R. H. COLBORNE ELECTRON DISCHARGE DEVICE INCLUDING FLUORESCENT SCREENS Filed Nov. 22, 1940 NVENTOR RONALD HENRY COLBOR/VE ATTORNEY Patented July 27, 1943 ELECTRON DISCHAR GE DEVICE, INCLUDING FLUORESCENT SCREEN Ronald Henry Colborn assignor to Electric itcd, Hayes, Middles Great Britain Application November e, West Drayton, England, & Musical Industries Limex, England, a company of 22, 1940, Serial No. 366,563

In Great Britain November 23, 1939 Claims. (Cl. 250-164) The present invention relates to electron discharge devices such as cathode ray tubes which include a screen which in operation is excited to luminescence by electron bombardment.

In such devices, for example, in a cathode ray tube having fluorescent screen it is usually desirable that under electron bombardment the screen shall attain a potential substantially equal to that corresponding to the highest velocity attained by the bombarding electrons. Should this not be the case, and the screen voltage stabilises itself at a lower potential than that of the final anode, a loss of brightness occurs due to the slowing down of the electrons immediately in front of the screen and positive ions formed by the electron discharge in front of the screen will impinge on the screen causing a local, but permanent, diminution of the fluorescent efficiency of the screen. 1

In order to ensure that, in operation, the screen shall be maintained at the desired voltage, fluorescent screens or cathode ray tubes have been provided with a metal backing which may be transparent or opaque and which is maintained at a potential equal to that of the final accelerat-.

ing electrode. A further method of achieving the desired result is to coat the screen with a metal, usually one having a low work function such as caesium, the usual method of coating being to distil the metal on to the screen from a side tube communicating with the interior of the discharge tube, in which the screen is mounted.

The object of the present invention is to enable the secondary emission from a luminescent screen in an electron discharge device to be improved without modifying the form of the envelope of the discharge device, for example, by connecting of a side tube, and even without including a special sensitising stop in the manufacture of the screen, the screen being sensitised during the normal baking process for the tube.

According to one feature of the invention a method of manufacturing an electron discharge device having a screen which, in operation, is excited to luminescence by electron bombardment is provided, said method including the step of introducing within the envelope of said device in such position that access to the screen is assured, a material comprising or including a chemical compound which on heating will give rise to a deposit on said screen which renders the screen emissive of secondary electrons when in operation, and heating said material to form said deposit. The material used in accordance with the above feature of the invention may be of such a nature that it will distil during the ordinary baking process for the tube or as a result of separate moderate heating and can be applied directly to the inside of the wall of the tube in such position that good access to the screen is assured, or the material can be mounted on a metal plate which can be heated by high frequency induction to cause the material to distil onto the screen. The material may also be included in an anodic coating applied to the wall of the tube. The material used for rendering the screen emissive of secondary electrons may be, for example, a hydroxide, halide or nitrate of an alkali metal, these materials being capable of being readily handled and being known to have good secondary emission ratio.

In accordance with a still further feature of the present invention, a method of manufacturing an electron discharge device having a screen which, in operation, is excited to luminescence by electron bombardment is provided, said method including the step of rendering said screen secondary-electron-emissive by including in the screen mixture or applying to the screen a material which, when heated, is reduced and, on reduction, yields a material having the desired effect, and heating said material to reduce it. Suitable materials are, for example, the inorganic salts referred to above and organic salts of alkali metals, such as the oxalates and acetates which, on heating, are reduced to the oxide of the metal.

The method of carrying the invention into effect will be readily understood from the following description in which reference is made by way of example to the accompanying drawing, in which each of the three figures is a diagrammatic representation of the envelope of a cathode ray tube showing a particular method of introducing a composition for rendering the screen emissive of secondary electrons.

In each of the figures of the drawing l indi cates the envelope of the cathode ray tube generally, 2 is the neck, and 3 is the frustro-conical portion of the tube on the end 4 of which the screen is supported in well understood manner. While the housing or envelope I has been represented as being of conventional shape, it will be realised, of course, that the invention is applicable to devices having housing of other than conventional shape, for example, cylindrical.

In the case shown in Figure 1, the composition introduced in accordance with the invention is applied on the inside wall of the tube in the form of an annulus 5, disposed near the end 4 of the 9 in the tube.

tube. The amount of material required to form the annulus is a few milligrams, the material being, for example, one of the alkali hydroxides, halides or nitrates. Preferably, the material is applied in the form of a solution or mixed with a suitable binder.

In Figure 2, the material is represented as appliedto a single rectangular area 6 within the tube.

In the further alternative represented in Figure 3, the material to be vaporised is supported on the metal tab 1 carried on wire 8 mounted in nipple In this case the material is vaporlsed by heating tab 1 by eddy current heating.

The material having the desired sensitising action on the screen may also be applied with the second anode coating, the coating lying between the lines [0 shown in Figure 1. In this case the screen sensitising material is preferably included in the part of the anode coating lying within the l enlarged portion 2 of the tube.

The sensitising material may also be included in the mixture applied to the end 4 of the tube to form the screen of the tube.

I claim:

1. The method of improving the operation of a cathode ray tube wherein is provided a luminescent screen which comprises the steps of depositing within the tube in the vicinity of said screen an alkali compound chosen from the group of radicals consisting of hydroxides, halides, nitrates, and organic salts reducible to oxides, and depositing a layer of said alkali compound upon the luminescent screen by baking said tube.

2. The method of improving the operation oi a cathode ray tube wherein is provided a luminoscent screen which includes the steps of depositing within the tube on the wall thereof adjacent to the screen an alkali compound chosen from the group of radicals consisting of hydroxides, halides and nitrates, and baking said tube to deposit a layer of an oxide of the alkali from the compound upon the luminescent screen.

3. The method of improving the operation of a cathode ray tube wherein is provided a, luminescent screen which comprises the steps of depositing an organic salt of an alkali metal, which salt is reducible to the oxide of the metal upon the luminescent material, and producing a layer of oxide on the luminescent material by baking the tube.

4. A cathode ray tube comprising an envelope having an end wall, a layer of luminescent material upon said end wall, and a relatively thin layer of an alkali compound of radicals consisting of hydroxides, halides and nitrates within said tube adjacent the said layer of luminescent material.

5. A cathode ray tube comprising an envelope having anend wall, a layer of luminescent material upon said end wall, and a relatively thin layer of an alkali metal organic salt capable of being reduced to the oxide of said alkali metal deposited upon said luminescent material within said tube adjacent the said layer of luminescent material.

RONALD HENRY C OLBORNE. 

